Alloys containing a high-nickel such content as inconel(Ni-Cr), hastelloy(Ni-Cr-Mo), and incolloy have been becoming widely employed because of its superior heat resistance and corrosion resistance. Recently there is an increasing demand for improving wear resistance and other properties of a alloy containing nickel and expanding its applicable fields. However, as to the above-mentioned nickel alloys such as inconel, the method for the improvement of surface hardness has not been established yet. A method of push-out hardening for the improvement of base material intensity and a use of superplastic articles employing powder material are merely studied. However, since the method of push-out hardening increases stiffness of whole alloy, workability of the alloy is damaged. Also superplastic articles employing powder material have difficulty in putting them in practical use due to extremely high cost.
By the way, the mainstreams of the methods of surface hardening for general metallic materials are as follows, 1 a plating method, 2 a coating method such as PVD, and 3 a diffusion method such as nitriding and boriding. However, as to nickel alloy, only some of the coating methods, such as full hard chrome plating and alumina coating, are partly put in practical use as mentioned above. Those methods have difficulty in quality control which is peculiar to the coating method, and limit the application range due to the thinness of the coating. Moreover, high cost for treatment is another problem. As the diffusion method of surface hardening, plasma ion nitriding using glow discharge has been also tried partly as to inconel alloy and hastelloy alloy. However, the treatment of such plasma ion nitriding scarcely forms a hardened nitrided layer on the above-mentioned nickel alloy. Even if formed, that would be a partly formed ultra thin layer with only several micron depth. Therefore, in the present situation, nitriding the above-mentioned nickel alloy is almost given up and far from turning it to practical use.
Accordingly it is an object of the invention to provide a method of nitriding nickel alloy for the improvement of a the surface hardness of the nickel alloy by which a uniformly nitrided deep layer can be formed on the nickel alloy surface.